With the increasing development of the air conditioning technology, the DC variable-frequency air conditioner draws more and more attentions. Currently, all conventional variable-frequency air conditions need dedicated installation, and the indoor apparatus and the outdoor apparatus need special communication mode so as to control the compressor in the outdoor apparatus.
A structure of a conventional variable-frequency air conditioning system is shown in FIG. 1A. The outdoor apparatus receives a series of signals based on the specific transmission rule and sent from the indoor apparatus, including the on-off signal, the temperature determination signal and the indoor temperature signal, such that the compressor in the outdoor apparatus can be controlled to operate with certain frequency and the working frequency of the compressor can be adjusted according to the indoor temperature. In order to implement the control of each element in the outdoor apparatus, the indoor apparatus is required to send corresponding signals to the outdoor apparatus, such that the air conditioning system has disadvantages of complex structure and high cost.